From the Tools menu, select Customize and then on the Commands tab of the
Customize dialog, select the All Commands category and scroll down through
the list of commands until you come to the Mail Merge Helper item.  Click on
it and drag it onto a toolbar (I would suggest the Mailmerge toolbar that
you can cause to be displayed by selecting Toolbars from the View menu and
checking it).  Then with the mailmerge main document as the active document,
if you click on the Mail Merge Helper button, in Section 2 of the dialog
that appears, you will be able to detect the datasource.